Respiratory systems provide breathable gas, such as oxygen, anesthetic gas and/or air directly to a patient's mouth, nose or airway to assist or facilitate breathing by the patient. A ventilator may be used as part of the respiratory system to drive the breathable gas to the patient through an inspiratory limb hose or conduit of a breathing circuit. An expiratory limb hose or conduit of the breathing circuit is provided to carry expelled air and other gas(es) from the patient back to the ventilator.
It is typically desired to warm and impart humidity to the breathable gas before it is provided to the patient. For that purpose, many respiratory systems include a humdificiation system having a chamber for holding water and a heater unit to which the chamber may be releasably mounted. The heater unit includes a heater, which may be comprised of one or more heating elements and a metal plate defining a hot plate. A wall of the chamber, such as the bottom surface of the chamber, is thermally conductive and in thermal contact with the hot plate of the heater, to thus heat the water in the chamber. The chamber may be manually refillable, or there may be a water source to selectively fill the chamber as it empties. The breathable gas is coupled to the chamber and is passed through the chamber to be heated and humidified. Examples of heater unit and chamber arrangements are shown in U.S. Pat. Nos. 6,988,497 and 5,943,473. The inspiratory limb carries the heated and humidified gas to the patient and the expiratory limb carries exhaled air and possibly other gases from the patient.
As the gas(es) travel to and from the patient along the elongated hoses or conduits of the inspiratory and expiratory limbs, respectively, they tend to cool such that condensation can form in the limbs. This condensation, referred to as rainout, can present serious and sometimes severe hazards. When present, the rainout must be carefully drained and handled. To reduce the incidence of rainout in the inspiratory limb, a heater circuit is provided to add heat to the heated and humidified gas passing through that limb to the patient. The heater circuit may be in the form of one or more elongated, and possibly coiled, heater wires running along the limb, such as through the interior of the limb. The temperature of the gas at the patient, such as at the outlet of the inspiratory limb, is measured, and a power circuit is operated to control the heater circuit in an effort to add heat as necessary to achieve a desired or set point temperature of the gas thereat.
Typically, the set point is selected to be above the outlet temperature of the chamber. Also, by heating the gas as it passes through the inspiratory limb, the incidence of rainout can be reduced. Where an expiratory limb is also provided, however, cooling of the expelled gas(es) passing through that limb increases the incidence of rainout in the expiratory limb. Hence, the expiratory limb may also include a heater circuit coupled to the power circuit so that the gas(es) passing through the expiratory limb is also heated. Heating the expiratory limb advantageously helps reduce rainout in the expiratory limb. An example of a breathing circuit with heated limbs is shown in U.S. Pat. No. 6,078,730.
The heater circuits of the two limbs may be substantially identical, such that the temperature in each limb is being driven to the same internal temperature. However, undesirable levels of rainout might still occur in the expiratory limb because the inspiratory limb is being controlled to deliver a specific airway temperature. Accumulation of fluid in the expiratory circuit as a result of such rainout can cause increased expiratory resistance and lead to unintentional lavage or fluid overload of the patient's lungs. Expiratory limb rainout may be further reduced by maintaining a higher temperature in the expiratory limb than in the inspiratory limb, such as an offset or temperature differential of 4 to 5 degrees Celsius. To achieve such a temperature differential, it is proposed to use different watt density heater wire in one limb than is used in the other limb. As a result, operation of the power circuit results in different levels of heating within the respective limbs aimed at maintaining a fixed temperature differential therebetween. Using different watt density wires for the respective limbs thus has advantages, but not without drawbacks. For example, use of different watt density wires creates inventory and manufacturing issues. Further, the range of operating temperatures in the two limbs may be limited.